The present disclosure relates generally to backlight units used as an illumination source for a display device and, more specifically, to backlight units having light-emitting elements being configured to provide a degree of redundancy in the event that one or more of the light-emitting elements malfunctions during operation.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the subject matter described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, not as admissions of prior art.
Electronic devices increasingly include display devices to provide visual feedback as part of a user interface. For instance, display devices may display various images associated with the operation of the electronic device, including photographs, video, text (e.g., a document, a webpage, or an e-mail, etc.), as well as images associated with a graphical user interface (e.g., icons, windows, screens, etc.) of the electronic device. As may be appreciated, display devices may be employed in a wide variety of electronic devices, such as desktop computer systems, laptop computers, and handheld computing devices, such as cellular telephones and portable media players. In particular, liquid crystal display (LCD) panels have become increasingly popular for use in display devices, due at least in part to their light weight and thin profile, as well as the relatively low amount of power required for operation.
However, because an LCD does not emit or produce light on its own, a backlight unit is typically provided in conjunction with the LCD panel as part of the display device in order to produce a visible image. A backlight unit typically provides backlight illumination by supplying light emitted from one or more light-emitting elements (a light source) to the LCD panel. Light-emitting elements commonly used in backlight units may include cold cathode fluorescent lamps (CCFLs) or light emitting diodes (LEDs). For example, backlight units utilizing LEDs may include one or more groups of LEDs, referred to sometimes as strings.
It is generally inevitable that a percentage of manufactured LCDs may become defective during their operational lifetime due, for example, to one or more of the light-emitting elements of the backlight unit malfunctioning. When this occurs, the affected light-emitting elements may become inoperable and cease emitting light, thus reducing the amount of light that may be provided by the backlight unit. From the perspective of a user, this may result in a noticeable reduction in the brightness in some parts or all of the screen of the LCD, which may cause images displayed on the screen to appear dimmer than intended or, in some cases, completely unperceivable, such as in a scenario in which all of the light-emitting elements of the backlight malfunction. Unfortunately, it is generally difficult and sometimes cost-prohibitive to repair LCDs in the event of such a malfunction.
There are currently two ways to make white light with LEDs: one method uses multiple wavelengths from different LEDs to make white light (e.g., a red LED, a green LED, and a blue LED), and the second method uses a white LED (e.g., a blue Indium-Galium-Nitride (InGaN) LED with a phosphor coating which creates white light). With regard to the second method, most manufacturers of high-power white LEDs estimate a lifetime of around 30,000 hours at the 70% lumen maintenance level, assuming maintaining junction temperature at no higher than 90 degrees Fahrenheit. Therefore, white LED failures may occur when LED junction temperature rises above this temperature.
LED backlighting employs different schemes—one of which is an edge lit scheme. In an edge lit scheme, a light bar (or light source) may be mounted along an edge of the display to deliver light into a light guide that diffuses light evenly across the display. This edge lit scheme has its advantages in terms of cost, compactness and very flat modular construction of the backlight. However, when a string of LEDs is used to deliver light into the light guide, some additional space (sometimes referred to as “mixing distance”) is used to allow for light from the individual LEDs to diffuse or mix, and this mixing distance usually depends on the distance between adjacent LEDs. Beyond this mixing area, homogeneous or mixed light is available for illuminating the display.